Epoxy resins have been used in a wide variety of applications such as coating applications, for example, because of their superiority in corrosion resistance, chemical resistance and flexibility. In order to improve workability, the epoxy resins are usually diluted with an organic solvent to decrease their viscosities.
Recently, various laws and regulations regarding use of organic solvents, such as organic solvent intoxication prevention regulations and fire regulations, have become more stringent. Because of such regulations, it is desired to make epoxy resins for coating compositions water soluble or water dispersible. Several attempts have been made to modify conventional epoxy resins which per se are not water soluble. For example, self-dispersion type epoxy resins modified with an acrylic resin have been proposed and disclosed in Japanese Patent Application Laid-Open Gazettes Nos. Sho 55-003,481 and Sho 58-198,513.
Further, for simplification of steps, studies have been carried out to make an epoxy resin per se water dispersible. For example, in Japanese Patent Application Laid-Open Gazette No. Hei 2-212,570, an epoxy resin is reacted with phosphoric acid to make the resin water dispersible. However, phosphoric acid has three hydroxyl groups and thus triesters of phosphoric acid are easily formed. For this reason, to obtain an epoxy phosphate ester resin having low viscosity and good dispersion stability, excess amounts of phosphoric acid must be employed. Therefore, unreacted phosphoric acid may remain in the resin product resulting in problems such as blushing of resultant coatings.
Japanese Patent Publication Gazette No. Hei 1-055,299 discloses an epoxy phosphate ester resin prepared by reacting an epoxy resin with phosphoric acid, and then hydrolyzing the phosphoric acid modified epoxy resin for monoesterification. The resultant resin per se can be made water dispersible. However, the resins have unduly large particle size when dispersed in water, resulting in poor dispersion stability. Accordingly, if the amount of organic solvent used is restricted, sufficient dispersion stability cannot be obtained. In particular, in paints prepared using an aromatic epoxy resin, a completely transparent aqueous solution cannot be obtained. In the case of relatively low molecular weight epoxy resins having a weight average molecular weight of 340 to 4,000, there is a problem that they are difficult to hydrolyze and their viscosities increase when dispersed in water.
In view of the deficiencies of the prior art described above, it would be desirable to provide an epoxy phosphate ester resin which exhibits good water-solubility, and its production process.
It would also be desirable to provide a coating composition having good coating properties such as a good anti-rust property and good flexibility.